Over the years, felines have preyed on their share of the songbirds he does want. One spring was particularly harrowing: A cat killed a nesting robin and a nesting wren.

"People just think cats have the right to be outside, but they're about the only species allowed to run free," said Marra.

Scientists are quietly raging about the effects that cats, both owned and stray, are having on bird populations. It's not an issue that has received much attention, but with an estimated 90 million pet cats in the United States, two-thirds of them allowed outdoors, the cumulative effect on birds is significant, according to experts.

"In the recent oil spill, fewer than 10,000 birds were killed in the Gulf [of Mexico] that we know of," said Steve Hutchins of the Bethesda-based Wildlife Society. "But literally millions of migratory birds are killed every year by feral and free-roaming pet cats. It's a serious environmental problem."

This is, as Marra realizes, "a charged issue." For gardeners seeking to attract cardinals, chickadees and goldfinches with feeders, baths and bird-friendly plantings, the sight of a neighbor's cat stalking and killing these feathered friends can be extremely upsetting. Cat owners, however, believe their pets need to be outside and that having a bell on their collar will warn birds of their approach.

Experts say that the birds don't heed the bells or that cats learn to keep them silent. Either way, collar bells "can reduce their hunting effectiveness a little bit, but not that much," said Caren Cooper, a research associate at the Cornell Lab of Ornithology.

Threatened species?

Scientists trying to quantify this problem have come up with some alarming figures.

The number of pet cats in the United States has tripled in the past four decades, and each outdoor cat kills between four and 54 birds a year, according to wildlife biologists Nico Dauphiné and Robert J. Cooper in a review paper published last year by the bird conservation consortium Partners in Flight. They estimated that at least one billion birds are killed by cats annually, "and the actual number is probably much higher."

"Two-thirds of all bird species are in decline in the U.S.," said Steve Holmer, a policy adviser with the American Bird Conservancy in Washington. "Cats are a contributing factor."

Elizabeth Parowski, the spokeswoman for Alley Cat Allies, says there is no evidence that cats are affecting overall bird populations or species. It's humans, not felines, that are the leading cause of bird mortality, said Parowski, whose Bethesda-based group is a national advocacy organization for stray cats. Tens of millions of birds die from being hit by cars or flying into buildings.

"Another 67 million [die] from exposure to pesticides," she wrote in an e-mail. "These figures do not even begin to take into account the direct impact of habitat loss. In metropolitan Washington alone, suburban development has destroyed thousands of acres of wildlife habitat over the last 15 years."

Bird advocates, however, believe that if the public had a better understanding of the scale of the bird carnage, there would be less willingness to let cats roam at will.

Nancy Peterson, the Humane Society of the United States' cat programs manager, said "the cat-versus-the-birds debate is a real one, [but] we feel it's really unproductive to continue the debate." She said it's "unfeasible" to expect the issue of feral cats to go away. But for pet cats, "they should be indoors or sufficiently supervised on their property," she said.

Baby catbirds study

For Marra, the problem goes beyond a personal interest. As a research associate at the Smithsonian Conservation Biology Institute, he recruited a network of backyard birders named Neighborhood Nestwatch to help study the fortunes of baby catbirds in neighborhoods in Bethesda and Takoma Park.

The researchers picked neighborhoods with dense and light cat populations and tracked spring nestlings with radio transmitters. In the neighborhoods with a lot of cats, 80 to 90 percent of the fledglings died, most of them killed by cats. In the other areas, about 50 percent died, a rate found in nature, he said. The results suggest that catbirds in cat-heavy areas are not able to reproduce at a rate that is sustainable. The predators "aren't feral cats; they're domestic cats allowed to go outside," he said. "And this is just one [bird] species."

Cornell's Cooper said that of the 836 species protected under the Migratory Bird Treaty Act, "about a quarter are in some kind of trouble." Two summers ago, she saw a cat take two fledgling mourning doves whose progress she had followed as nestlings.

What you can do

Wildlife biologists say that the remedy is to keep pet cats indoors. This is healthier for the cat as well as the birds, they say. Cats that roam outside are more likely to suffer injuries from cars, dogs and other cats, to contract life-threatening diseases and to become prey themselves, they say.

Hutchins said that if cat owners want to have their pets in the garden with them, as many do, they can be kept on a leash or contained in enclosed cat runs.

Strays and out-and-out feral cats are also a significant problem for birds. Dauphiné and Roberts say there may be as many as 100 million such cats in the country. They argue that increasing efforts to control feral cat colonies through sterilization rather than euthanasia don't seem to be reducing numbers, largely because new migrants join the colonies.

Of course, turning an outdoor cat into an indoor cat is not as easy as it sounds. Sue Mandeville, a retired university employee and gardener in Springfield, Ore., said she tried to keep her three cats inside after they started catching birds. "They became very angry at me," she said, and showed it by marking parts of the house with urine. The standoff lasted three weeks, and "everyone was miserable."

Mandeville gave up on keeping her cats under house arrest; wondering how she could have them outside without killing birds, she came up with the idea of a bib. She cut up the leather tongue of her husband's boot and attached it like a shield hanging from the cat's collar. She refined the design and now uses two-millimeter-thick neoprene bibs that weigh "less than a chickadee," she said. "It gently interferes with their timing and coordination," she said. "They can still climb trees and jump on counters and run," but it makes them much less dangerous to birds. She sells the CatBib from her Web site, http://www.catgoods.com.